Health effects of tea
The health effects of tea have been examined ever since the first infusions of Camellia sinensis about 4700 years ago in China. The legendary emperor Shennong claimed in The Divine Farmer's Herb-Root Classic that Camellia sinensis infusions were useful for treating conditions including tumors, abscesses, bladder ailments, and lethargy.
Historically, it was probably beneficial in many situations before modern safe drinking water that tea is normally made with boiled water, with at least a reduced load of microrganisms. In addition tea itself has significant antimicrobial properties, against for example vibrio cholerae, the bacterium responsible for cholera.
Tea contains catechins, a type of antioxidant. In a freshly picked tea leaf, catechins can compose up to 30% of the dry weight. Catechins are highest in concentration in white tea and green tea, while black tea has substantially fewer due to its oxidative preparation. Research by the U.S. Department of Agriculture has suggested that levels of antioxidants in green and black tea do not differ greatly, with green tea having an oxygen radical absorbance capacity (ORAC) of 1253 and black tea an ORAC of 1128 (measured in μmolTE/100g). The amounts of carbohydrates, fat, and protein found in tea are negligible. Although tea contains various types of phenolics and tannin, tea does not contain tannic acid. Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.
Theanine and caffeine
Tea also contains theanine and the stimulant caffeine at about 3% of its dry weight, translating to between 30 mg and 90 mg per 8 oz (250 ml) cup depending on type, brand and brewing method. Tea also contains small amounts of theobromine and theophylline. Due in part to modern-day environmental pollution fluoride and aluminium have also been found to occur in tea, with certain types of brick tea made from old leaves and stems having the highest levels. This occurs due to the tea plant's high sensitivity to and absorption of environmental pollutants. Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations—which results in a cup of brewed tea containing significantly less caffeine than a cup of coffee of the same size.
Green tea may lower blood low-density lipoprotein and total cholesterol levels, though the studies were of short duration and it is unknown whether these effects result in fewer deaths; moreover, the evidence does not indicate that green tea reduces the risk of coronary artery disease. Several randomized controlled trials suggest green tea can reduce body fat by a small amount for a short time, though it is uncertain whether the reduction would be meaningful for most people.
The Food and Drug Administration (FDA) in 2005 reports that green tea consumption has no known beneficial effect on breast or prostate cancer. "Two studies do not show that drinking green tea reduces the risk of breast cancer in women, but one weaker, more limited study suggests that drinking green tea may reduce this risk. Based on these studies, FDA concludes that it is highly unlikely that green tea reduces the risk of breast cancer."
The FDA also finds that existing evidence does not support qualified health claims for green tea consumption and a reduced risk of any other type of cancer.
The US National Cancer Institute reports that in epidemiological studies and the few clinical trials of tea for the prevention of cancer, the results have been inconclusive. The institute "does not recommend for or against the use of tea to reduce the risk of any type of cancer." ... "Inconsistencies in study findings regarding tea and cancer risk may be due to variability in tea preparation, tea consumption, the bioavailability of tea compounds (the amounts that can be absorbed by the body), lifestyle differences, and individual genetic differences."
Though there is some positive evidence for risk reduction of breast, prostate, ovarian and endometrial cancers with green tea, it is weak and inconclusive.
All tea leaves contain fluoride; however, mature leaves contain as much as 10 to 20 times the fluoride levels of young leaves from the same plant. Although low concentrations of fluoride are maintained in many public water supplies for dental health, very high fluoride intake (over 2 mg per day for children, 4 mg adults) increases the risk of osteofluorosis and fractures. There is evidence that over-intake of teas produced using mature leaves (e.g. brick tea) or a combination of mature and young (e.g. through inefficient mechanical harvesting) can cause fluorosis in humans. 
The fluoride content of made tea depends on the picking method and fluoride content of the soil in which it is grown; tea plants absorb this element at a greater rate than other plants. Care in the choice of the location where the plant is grown may reduce the risk. It is speculated that hand-picked tea would contain less fluoride than machine-harvested tea, because there is a much lower chance of harvesting older leaves during the harvest process. A 2013 British study of 38 teas found that cheaper UK supermarket tea blends had the highest levels of fluoride with about 580 mg per kilogram, green teas averaged about 397 mg per kg and pure blends about 132 mg per kg. The researchers suggested that economy teas may use older leaves which contain more fluoride. They calculated a person drinking a litre of economy tea per day would consume about 4 mg of fluoride, the maximum recommended amount of fluoride per day but below the maximum tolerable amount of 10 mg fluoride per day.
Aluminum and heavy metals
Tea drinking accounts for a high proportion of aluminum in the human diet. The levels are safe, but there has been some concern that aluminum traces may be associated with Alzheimer's disease. A recent study additionally indicated that some teas contained possibly risky amounts of lead (mostly Chinese) and aluminum (Indian/Sri Lanka blends, China). There is still insufficient evidence to draw firm conclusions on this subject.
The caffeine in tea is a mild diuretic. However, the British Dietetic Association has suggested that tea can be used to supplement normal water consumption, and that "the style of tea and coffee and the amounts we drink in the UK are unlikely to have a negative effect [on hydration]".
Tea contains oxalate, overconsumption of which can cause kidney stones, as well as binding with free calcium in the body; other minerals may be bound as well. The bioavailability of oxalate from tea is low, thus negative effect requires a large intake of tea.
Hot drinking temperature
A study, published in 2012, suggested that men who drink large quantities of black tea – more than seven cups per day – increase their risk of prostate cancer by 50%. The story was relayed in the media, but according to the National Health Service, "this study had many limitations that call into question the reliability of its results".
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